upper esophageal sphincter

The UES is composed mainly of the cricopharyngeus muscle along with contributions from the inferior pharyngeal constrictor and the proximal esophageal circular muscle layer. Unlike the lower esophageal sphincter, the UES consists of skeletal (striated) muscle and is under voluntary and reflexive neural control via the vagus nerve (nucleus ambiguus) and swallowing center in the medulla. At rest, it maintains a high-pressure zone (~40-100 mmHg) to prevent air from entering the esophagus during respiration and to prevent reflux of esophageal/gastric contents into the pharynx and airway (protecting against aspiration). During swallowing, the UES relaxes in a coordinated fashion with pharyngeal contraction and laryngeal elevation, allowing the bolus to pass into the esophagus, then it contracts again. Clinically, UES dysfunction is implicated in cricopharyngeal achalasia, Zenker's diverticulum (a false diverticulum that forms due to increased intraluminal pressure against a poorly relaxing or spastic cricopharyngeus, typically at Killian's triangle), and oropharyngeal dysphagia often seen in neurological conditions (stroke, ALS, Parkinson's disease, myasthenia gravis) affecting the swallowing reflex arc. Diagnostic evaluation includes videofluoroscopic swallow study (modified barium swallow) and esophageal manometry. Treatment of UES dysfunction may involve cricopharyngeal myotomy, balloon dilation, or Botox injection in refractory cases.